The invention relates to electric motors and, in particular, to motor constructions which have a preferred direction of rotation and are prevented from rotation in an opposite direction.
Electric motors of certain types such as single phase motors of the permanent split capacitor type may have a tendency when electrically energized to maintain the direction of rotation that they have before energization. Fans, impellers, pumps and other machine elements may have a tendency to back drive or windmill a motor in a direction opposite that for which it was designed. Thus, when the back driven motor is electrically energized, it continues to rotate in the wrong direction.
U.S. Pat. Nos. 3,821,567 and 4,716,325, for example, show mechanisms that ensure that a motor operates from a start-up in a preferred direction. The first of these patents relies on axial displacement of a rotor when rotation occurs in the wrong direction; it has the potential for malfunction because it relies on a differential in frictional forces that may be adversely affected by dirt, moisture, or other environmental conditions. The second of these patents has a displaceable arm that can block rotor rotation in the wrong direction. The arm requires some type of separate electrical actuation and, therefore, can add significantly to the complexity and cost of the motor.